1. Field of the Invention
The present invention relates to a method and an apparatus for determining whether a workpiece has been successfully or has failed to be press fitted by the thrust of a plunger linearly driven.
2. Description of the Prior Art
FIG. 4 is a diagram showing a configuration of a press fitting system 90 using a conventional determining apparatus 80, and FIG. 5 is a diagram for explaining the operation of the determining apparatus 80. In FIG. 5, the ordinate represents the thrust LD of a cylinder CY, i.e. the press fitting force of a plunger PR, and the abscissa represents the position LS of the plunger PR.
As shown in FIG. 4, the press fitting system 90 includes the cylinder CY for press fitting, a load cell 81, a position sensor 82, amplifiers 83, 84, a peak hold unit 85, a setting unit 86, a determining unit 87 and a sequencer 88. The plunger PR is mounted at the forward end of a piston rod PD of the cylinder CY. An object member BZ is arranged under the cylinder CY.
With a workpiece WK placed at a press fitting position AP of the object member BZ, the cylinder CY is driven to move the plunger PR downward. In this way, the workpiece WK is press fitted into the object member BZ. The object member BZ has a contact end KT adapted to contact the workpiece WK. Until the lower end surface of the workpiece WK comes into contact with the contact end KT, the plunger PR moves along a press fitting section 81 extending from a press fitting start position LSI to a press fitting end position LSE.
The load cell 81 detects the thrust LD of the cylinder CY, i.e. the force of the plunger PR for press fitting the workpiece WK. The amplifier 84 amplifies the output signal of the load cell 81 and outputs a thrust signal S81.
The position sensor 82 detects the position of the piston rod PD of the cylinder CY thereby to detect the position LS of the plunger PR and the workpiece WK. The amplifier 83 outputs a position signal S82 in response to the output of the position sensor 82.
The peak hold unit 85, as shown in FIG. 5, detects and holds a maximum peak thrust signal LDX and a minimum peak thrust signal LDN on the thrust force curve S81 in a press fitting force determining section R82.
The setting unit 86 has preset therein a maximum peak thrust reference value FH, a minimum peak thrust reference value FL and a thrust reference value FK at the contact end.
The determining unit 87 compares the maximum peak thrust reference value FH with the maximum peak thrust signal LDX and the minimum peak thrust reference value FL with the minimum peak thrust signal LDN. Also, the determining unit 87 compares the thrust signal LDK at the press fitting end position LSE with the thrust reference value FK at the contact end.
In the case where the thrust signal LDX is not more than the thrust reference value FH and the thrust signal LDN is not less than the thrust reference value FL, it is determined that the workpiece WK has been press fitted under a predetermined press fitting force. Also, in the case where the thrust signal LDK is not less than the thrust reference value FK, it is determined that the lower end surface of the workpiece WK has come into contact with the contact end KT. Once these two conditions are met, it is determined that the result of press fitting is "successful", and the control signal CS1 to that effect is output to the sequencer 88. In accordance with the control signal CS1, the sequencer 88 outputs a command required for the next press fitting session to the cylinder CY, etc.
In this way, in the conventional determining apparatus 80, the success or failure of press fitting is determined based on the maximum peak thrust signal LDX and the minimum peak thrust signal LDN in the press fitting force determining section R82.
For determination of the success or failure of the press fitting with a high reliability, it is desirable that the press fitting force determining section R82 is as near to the press fitting end position LSE as possible, i.e. the distance L81 between the determination end position LS3 and the press fitting end position LSE is as small as possible.
In the press fitting system 90, however, the sequencer 88 controls various equipment such as the conveyor for conveying the object member BZ as well as the cylinder CY. Therefore, the response to each equipment is not so high and takes a time as long as several ms to several tens of ms. The process of determining the success or failure of the press fitting also requires a time of about the same length.
Also, the workpiece WK and the object member BZ have a tolerance.
For these reasons, in the case where the distance L81 is too short in the conventional determining apparatus 80, the large load in the vicinity of the contact end KT may be picked up by mistake as a maximum peak thrust signal LDX.
In the prior art, therefore, the distance L81 is set to a comparatively large value, say, about 3 mm to 4 mm. With the increase in the distance L81, however, a press fitting failure is more liable to occur during the particular section, often resulting in a correspondingly higher determination error. In other words, the press fitting may be determined as a "success", even in the case where the actual press fitting force decreases and the thrust signal S81 decreases below the thrust reference value FL within the distance L81 as shown by a two-dot chain in FIG. 5.